1. Field of the Invention.
This invention relates to a rotary hammer with a pneumatic hammer mechanism having a reciprocable piston which drives a ram, which piston is driven by means of a driving part arranged rotatably on an intermediate shaft with which a first coupling element of a coupling non-rotatably connected, while a driven second coupling element of the coupling can be slid axially into positive engagement with the first coupling element, a synchronizing arrangement being provided in the coupling and arranged so that as the coupling elements are brought close together, the first coupling element is accelerated approximately to the rotational speed of the second coupling element.
2. Related Prior Art.
A known rotary hammer of this type (DE-Patent No. 31 36 264) has a first coupling element which is driven by the intermediate shaft by means of a gear arrangement and does not rotate when the hammer mechanism is not switched on. On the side of this first coupling element facing the tool-holder, there is a frusto-conical engagement surface, and in addition the first coupling element has coupling holes for the positive connection of coupling pins. The coupling pins are part of the second coupling element, which is in non-rotatable engagement with the rear part of the tool-spindle and rotates with it. A synchronizing disc is non-rotatably connected to the second coupling element, which is slidable axially on the rear part of the tool-spindle and is urged by a spring towards the first coupling element.
If the tool-spindle, following engagement with the work-piece of a tool inserted in the tool-holder, is displaced further into the housing of the rotary hammer then through the force of the spring acting on the synchronizing disc an outer frusto-conical surface provided on the synchronizing disc will be brought into engagement with the frusto-conical engagement surface of the first coupling element and accelerates this. Further inward displacement of the tool-spindle into the housing of the rotary hammer brings the front ends of the spring-loaded coupling pins near to the coupling holes of the first coupling element and finally into positive engagement with these coupling holes.
In this known rotary hammer it is possible, by the exertion of very strong pressure on the tool-spindle, for a very fast engagement of the coupling to take place, the result being that the first coupling element will not yet have been accelerated to approximately the rotational speed of the second coupling element when the front ends of the coupling pins begin to run over the coupling holes. This causes considerable noise and vibration, and when the pins suddenly enter the holes considerable vibration of the whole rotary hammer results.
Furthermore, it appears that the operation of the previously known coupling is questionable, because it cannot be understood how to prevent contact of the front ends of the coupling pins with the region of the first coupling element having the coupling holes immediately before or on light engagement of the respective frusto-conical surfaces of the first and second coupling elements. In view of this, proper synchronization cannot take place before the parts of the coupling provided for the positive coupling engagement come into engagement.
In another known rotary hammer (U.S. Pat. No. 3 430 709) in which the second coupling element is arranged to be non-rotatable on the intermediate shaft, the first and second coupling elements have teeth or dogs facing each other for positive coupling engagement, and the second coupling element is acted on by a spring operating in the direction of coupling engagement. The coupling elements can be held out of engagement by a linkage which can be operated externally so that when the linkage is released, the second coupling element is displaced axially under the action of the spring and comes into positive engagement with the first coupling element.
In this known rotary hammer, the engagement of the coupling elements can in practice only be achieved when stationary, i.e. the coupling must be brought into the desired position for the required operational mode before activation of the motor of the rotary hammer; that is, either in the position for simple drilling in which the two coupling elements remain out of engagement, or in the position for hammer-drilling action, in which the two coupling elements are in positive engagement. Switching-over between drilling and hammer-drilling with the intermediate shaft rotating leads to an impingement of the rotating teeth or dogs of the second coupling element non-rotatably mounted on the intermediate shaft with the stationary teeth or dogs of the non-rotating first coupling element connected with the driving part for the hammer action, and thus not only to a high noise production but also to considerable loading of the teeth or dogs and a sudden start-up of the hammer action, as a result of which heavy loadings occur inside the rotary hammer. Furthermore, it is quite uncertain when the positive engagement of first and second coupling elements actually takes place.
There is also already known (U.S. Pat. No. 4 280 359) a rotary hammer with a pneumatic hammer-mechanism in which, in order to switch to the hammer-drilling action, the spindle of the rotary hammer is displaced by the pressure of the hammer bit on the work-piece, and thus the second coupling element is brought into coupling engagement with the first coupling element. With this type of activation of the hammer action, in order to avoid difficulties because of the different rotational speeds of the rotating second coupling element mounted on the driven intermediate shaft and the non-rotating first coupling element connected with the initially non-rotating driving part for the hammer-mechanism, the two coupling elements have frusto-conical coupling surfaces of complementary shapes and which gradually come into engagement on axial displacement of the second coupling element so that the first coupling element is accelerated by the gradually increasing friction between the coupling surfaces and is gradually brought to the rotational speed of the second coupling element.
In this known construction, a force-determined or frictional coupling is used, i.e. a coupling which connects only through frictional engagement of the coupling surfaces. In use, therefore, the user must always exert such a pressure on the rotary hammer that the two coupling elements are kept in coupling engagement. Applying such a force makes the use of the rotary hammer more difficult, particularly because pulsating forces act on the coupling owing to the reciprocating movement of the piston of the hammer mechanism.